The use of cellular mobile telephones or mobile units is becoming increasingly common as the cost of these devices decrease. Mobile units can be divided into two major classifications: automobile-mounted units and fully portable handheld units. The operation of both types of mobile units within a prior art cellular phone network can be seen in FIG. 1, where the mobile units 11 and 13 may be placed in contact with other mobile units, or land line telephones via a Public Switched Telephone Network (PSTN) 15, by means of the cellular phone network 17. The cellular phone network 17 is comprised of a plurality of base stations 19 and one or more mobile telephone switching offices (MTSO) 21. Each base station 19 provides radio communication contact with the mobile units within a geographical area known as the base station coverage area 23. In sum, a cellular system can be thought of as one or more MTSOs 21, a plurality of base stations 19, and any number of mobile units, of which 11 and 13 are typical, that use the network. Generally, each of these components includes computerized controls in the form of hardware and/or software.
When a telephone call to a called mobile unit originates from either the PSTN 15 or a calling mobile unit within the cellular network, the caller must first access the cellular phone network 17. This is accomplished by dialing the called mobile units unique identification number, commonly known as its phone number. The MTSO 21 receives the call request and instructs a central call processor 25 to commence with call processing. The central call processor 25 instructs each of the base stations 19 to transmit a page in order to locate the mobile unit, e.g., mobile unit 11. A page is a signal that alerts a particular mobile unit that is being called. A page to one mobile unit is different in content from a page to any other mobile unit. For example, a page typically contains the paged mobile unit's identification or phone number. In this manner, pages to a particular mobile unit can be identified.
Each base station transmits the page on one or more dedicated forward control channels that carry all pages, as well as control signals, channel assignments, and other overhead messages to each mobile unit. The forward control channel is distinct from the other radio channels, which actually carry the voice communications between a mobile unit and another mobile unit or a land line telephone. Each base station may have more than one forward control channel upon which pages can be carried. For clarity however, this discussion will assume that each base station has one forward control channel.
Because a plurality of base stations are broadcasting at the same time, mobile units operating in the cellular system, and not currently engaged in a telephone call, a state typically referred to as being idle, will tune only to the strongest available forward control channel and monitor that channel for a page or other message directed to it. Upon determining that a page message is directed to it, mobile unit 11 again scans all forward control channels so as to select the base station transmitting the strongest signal. The mobile unit than transmits an acknowledgment message along a reverse control channel associated with the strongest forward control channel. This acknowledgment message serves to indicate to the MTSO 21 which of the several forward control channels (associated with the several base stations 19) to use for further call processing communications with mobile unit 11. This further communication typically includes a message sent to the mobile unit instructing it to tune to a particular voice channel for completion of call processing and for connection with the calling party.
It can be appreciated that in the cellular phone network described above, an idle mobile unit must constantly monitor pages transmitted on the forward control channel from the base stations in order to receive calls. This activity is necessary since messages addressed to a particular mobile unit may occur at essentially any point in the forward channel data stream. For the system to function satisfactorily, a mobile unit must be able to detect all pages addressed to it. Thus, idle mobile units must be "on" all the time, i.e., their electronic circuitry associated with receiver and base-band processing functions must be powered up continuously during idle mode. This power requirement arises from the high-speed digital modulation schemes used by cellular phone networks and known to those with skill in the art.
In the case of automobile-mounted units, power consumption is not a critical consideration since the automobile battery provides an adequate source of power. However, for the fully portable handheld mobile units with self-contained batteries, this power consumption may be highly undesirable since it limits idle mode battery life.
The general problem of reducing power consumption in a mobile unit while in idle state has been recognized for some time. Prior attempts to address this problem have resulted in varied solutions. However, the bulk of these solutions can be characterized as systems which organize the forward control channel data stream such that it consists of an infinite series of concatenated frames. Such systems are designed to lower the mobile unit power consumption during certain intervals by inactivating most high-power consumptive circuits. These intervals are referred to as the OFF periods or modes. During an OFF period, only a small timing circuit is active. This timing circuit is synchronized with the transmissions on the forward control channel being monitored. The mobile unit is periodically switched ON according to the state of the timing circuit to check for pages. If a page is recognized during an ON period, the mobile unit remains active until the call is completed.
To allow mobile units to only periodically monitor the control channel, each base station broadcasts frames of information on the forward control channel, each frame having an identical time span. This is called a frame oriented scheme. Every frame broadcast on a given control channel has precisely the same structure, i.e., each frame contains a unique data sequence, which those skilled in the art generally refer to as the "synchronization word", that is sent at precisely the same location in each frame. Typically, but not necessarily, the synchronization word is positioned at the beginning of each frame. The purpose of the synchronization word is to provide frame timing synchronization to mobile units receiving paging signals on the forward control channel. Within each of these frames are time slots, which are segments delineated by time during which data, such as a page, may be placed on the frame. Each frame may include control slots, which contain overhead messages and synchronization information, and a number of paging slots, which carry the actual pages. Each of the paging slots are located at a precisely defined time location with respect to the synchronization word.
In this prior art approach, each mobile unit in the cellular phone network is assigned a paging slot within the frames to receive its pages. Because there are a limited number of paging slots in each frame, it can be appreciated that each paging slot will have many mobile units assigned thereto. Each mobile unit, by virtue of some function program within it, is assigned to one, and only one, of these paging slots. This assignment for each mobile unit, which is often derived from a mobile unit's identification number, is also known to the system transmitting the paging channel. In practice, each of the paging slots may have tens of thousands of mobile units assigned to it. In operation, each mobile unit matches its identification number against the content of a page in its assigned paging slot to determine if the page is directed to it, or if the page is to another unit. Once a mobile unit is synchronized precisely with the base stations broadcasting of the frames, the mobile unit need only monitor the forward control channel during its assigned paging slot time period in each frame. During the remainder of each frame, the mobile unit may be in OFF mode to save power.
This type of message sending protocol is described in detail in U.S. Pat. No. 4,713,808. In this prior art improvement, when a message is sent to a particular mobile unit, rather than sending it at a random time, it is inserted into the mobile unit's assigned paging slot. As a result, each idle mode mobile unit receiving the paging channel, once it has required synchronization timing by receiving a synchronization word, can only monitor its assigned paging slot. At other times, the mobile unit may "turn off" its electronic circuitry with the exception of that required to maintain timing. This is commonly referred to by those skilled in the art as a mobile unit "sleep mode". The mobile unit "wakes up" in time to receive its assigned paging slot, then "goes back to sleep" if it receives no message in that slot.
While this prior art improvement greatly extends battery life, it nonetheless results in substantial and undesirable delays in transmission to the mobile units. More particularly, it is often the case that a paging channel needs to send messages for several mobile units at essentially the same time. If some of these messages are for mobile units that are assigned the same paging slot, there is no alternative but to send the messages in successive frames, thus delaying the transmission. As those skilled in the art will recognize, this results since each paging slot can only contain one page. In operation, one page is chosen, and the remainder of the pages for that paging slot will "stack up" at the base stations. This causes conflicting pages to be delayed one or more frame periods. As an example, given that a nominal frame length is approximately one second, and assuming that there are five pages to be sent to five different mobile units assigned a common paging slot, this paging method would cause a delay of five seconds before the last mobile unit is paged. This length of delay is undesirable.
Consequently, a need has developed for a method and system for overcoming the limitations of the prior art, and in particular, decreasing the response time of called mobile units while maintaining the power conservation that is so important in a portable unit. While the use of paging time slots has been broadly disclosed in the prior art on a time-domain multiplex communications channel, see for example U.S. Pat. No. 4,777,633 to Fletcher, the prior art does not teach or suggest the use of paging slots on a control channel, as disclosed herein by applicant.